Air is rectified into oxygen, nitrogen and argon component by a process known as cryogenic rectification. In such a process the air, or other gas containing oxygen, nitrogen and argon, is compressed, purified of higher boiling contaminants such as water vapor, carbon dioxide and hydrocarbons. At least part of the compressed and purified air is cooled to a temperature suitable for its rectification within a main heat exchanger. The cooled air is thereafter introduced into a double distillation column arrangement having a high pressure column thermally linked to a low pressure column to separate the nitrogen from the oxygen. An argon column can be connected to the low pressure column to receive an argon containing vapor stream from the low pressure column and to separate the argon from the oxygen. The separation produces an argon-rich stream as column overhead of the argon column.
The high and low pressure columns are thermally linked so that a nitrogen-rich vapor produced in the high pressure column is liquefied against a vaporizing oxygen-rich liquid column bottoms of the low pressure column. The nitrogen-rich liquid is used, at least in part, to reflux the high pressure column and also the low pressure column to initiate descending liquid phases within such columns. The descending liquid phases contact ascending vapor phases in mass transfer contacting elements such as sieve trays or structured packing so that the vapor phases become evermore rich in nitrogen as they ascend the columns and the liquid phases become evermore rich in oxygen as they descend within the columns. The ascending vapor phase in the low pressure column is produced by the vaporization of the oxygen-rich liquid and the ascending vapor phase in the high pressure column is produced from the incoming air. In the high pressure column an oxygen enriched liquid is produced that is known as kettle liquid. At least a portion of the kettle liquid is used to condense argon reflux in the argon column and then introduced into the low pressure column for further refinement. Oxygen and nitrogen-rich streams can be taken as products that are warmed in the main heat exchanger and help to cool the incoming air.
Since there exists heat leakage into such process and warm end losses in the main heat exchanger, refrigeration is imparted to the process. This is done by expanding some of the incoming air in a turboexpander or some of the nitrogen-rich vapor and introducing the resulting exhaust into the main heat exchanger. Typically, the nitrogen-rich vapor is extracted from the low pressure column. When air is used to generate refrigeration, the air separation plant is often referred to as an air expansion plant and when nitrogen is used from the low pressure column the plant can be termed as a nitrogen expansion plant. However, there have been air separation plants in which the nitrogen-rich vapor is expanded from the high pressure column.
Although in many schematic representations of air separation plants appearing in the open literature or in published patents and applications, the low pressure column appears to be the same diameter as the high pressure column. This is rarely if ever the case in practice. The reason for this is that the volumetric flow of vapor in the low pressure column is typically much higher than in the high pressure column due to its lower pressure. As the column diameter is reduced, the superficial velocities of the vapor and the liquid will increase due to the fact that volume flow is a function of a product of velocity and cross-sectional flow area. However, for a given reflux rate of liquid, there exists a limitation on the superficial vapor velocity produced by ascending vapor of the vapor phase at which the vapor resists the downflowing liquid progress in the column in a condition known as flooding. Thus, for a given reflux rate, the superficial vapor velocity must be less than the velocity that would otherwise produce flooding of the column. In order to manage the velocity in the low pressure column, the low pressure column requires a larger diameter than the high pressure column to decrease velocities in the low pressure column that would otherwise occur with the higher volumetric flow rates. In this regard, the superficial vapor velocity is not constant within the low pressure column and will vary in sections or regions thereof in which there are vapor feeds. However, there exists a maximum superficial velocity and the low pressure column diameter will be designed to accommodate the maximum without flooding in a manner very well known in the art.
The problem with having a low pressure column with a larger diameter than the high pressure column is that shipment costs of the column system from the fabricator to the site at which the plant will be erected are increased. The shipping girths can be such that shipping is impractical or cost prohibitive, and local fabrication becomes necessary. This is often not desirable as a result of high local labor rates and less controllable conditions. This being said, there is some degree of choice in the design of the low pressure column with respect to its diameter. For example, in a column using structured packing, in regions of the column at which there exists high superficial vapor velocities, a lower density packing can be used to reduce the diameter of the column. The problem with this is that the efficiency of the packing is reduced or in other words, the packing operates at a higher HETP resulting in the height of the column increasing. The increase in height also results in increased shipping and fabrication costs in that in such case the column is split into sections that are separately shipped.
As will be discussed, among other advantages, the present invention provides a method of conducting a cryogenic rectification process that incorporates a low pressure column that has substantially the same diameter, or even a lower diameter than the high pressure column. Moreover, this reduction in size effectuated without or with a minimum increase in height. The cryogenic equipment, including the distillation columns, heat exchangers, turboexpanders, with associated piping and valves is housed in a container called a cold box. A space is maintained between the cryogenic equipment and the cold box, and the cold box is internally insulated, often with dumped material such as perlite. The size of the cryogenic equipment directly affects the design of the cold box, its size, and is also important in project economics and time length related to the preceding shipping and fabrication discussion.